With an ever increasing shortage of mineral resources, low-grade high-ash fuel and raw materials with a high content of barren rock find expanding application in modern industry.
Where industrial furnaces and steam boilers operate on the foregoing types of raw materials and fuel, flue gases utilized at a later stage in air heating devices carry ash or other hard particles acting on heat-exchange tubes in said devices and bringing about heavy abrasive wear thereof.
The heat-exchange tubes undergo extensive wear because the flow of flue gases contracts at the inlet of the heat-exchange tubes and then expands with the results that the flow directed obliquely strikes the walls of the heat-exchange tubes in the air heating device. Since the flow of flue gases contains ash and other hard particles, the walls of heat-exchange tubes undergo heavy wear due to the action of said particles affecting the walls mostly in the area near the entry of flue gases into the heat-exchange tubes, that is, at a distance of two or three tube diameters from the inlet.
Wear of heat-exchange tubes greatly increases in modern high-power steam boilers and industrial furnaces wherein supply flues of large cross-sectional area incorporate bend portions inevitable under confined space conditions. The above-mentioned design of the supply flues does not permit constructing a sufficiently long vertical flue portion for sending the flue gas flow along the axes of the heat-exchange tubes, whereby the flue gas flow is not directed obliquely to the axes thereof and the rate of abrasive wear in the heat-exchange tubes is reduced. In oder to protect the heat-exchange tubes against abrasive wear, the supply flue of the air heating devices is commonly furnished with special-function devices.
Known in the art is a tubular air heater comprising heat-exchange tubes fastened to tube plates, and a supply flue (cf. USSR Inventor's Certificate No. 558133, F 23L15/04, published in 1974).
The prior-art air heater incorporates a device designed for protection of the heat-exchange tubes against abrasive wear, located in front of a tube plate on the side of the flue gas inlet with a certain clearance between the device and tube plate, and contrived in the form of a throttling perforated sheet wherein the axes of holes are aligned with those of the heat-exchange tubes.
When hot flue gases carrying hard particles or ash flow through the holes in the perforated sheet, the gas flow is divided into a number of jets that contract in the holes and expand at leaving them, with the result that the flow velocity decreases. The jets propagating at said velocity smoothly enter the heat-exchange tubes of the device all across the section thereof, and the rate of abrasive wear of the tubes decreases.
The above-mentioned air heater is applicable only where the vertical portion of the supply flue is long enough to direct the gas flow along the axes of the heat-exchange tubes. If flue gases are supplied through the supply flue wherein a bend section is installed, such air heater, is impracticable since the flue gas flow is directed at an acute angle to the surface of the perforated sheet. The perforated sheet per se cannot straighten the flue gas flow. Therefore the air heater heat-exchange tubes are exposed to heavy abrasive wear.
Besides, it should be noted that certain difficulties exist in manufacture and assembly of such air heater because the holes in the perforated sheet and heat-exchange tubes must be aligned to a high degree of accuracy.
There is also known a tubular air heater comprising heat-exchange tubes attached to tube plates, and a device installed in the supply flue for protection of tubes against abrasive wear and devised in the form of a perforated sheet carrying tubular insets fitted free into the holes thereof, with opposite ends of the insets introduced into the tube plate holes and fitted butt with the heat-exchange tubes (cf. USSR Inventor's Certificate No. 357419, F 23 L 15/04, published in 1972).
The foregoing prior-art air heater is applicable both when the flue gas flow is sent thereto through the vertical supply flue of when the gas flow is directed through the supply flue incorporating a bent portion. Since abrasive wear in the foregoing air heater is localized in the tubular insets, the heat-exchange tubes proper are protected against wear. It should however be noted that with the flue gas flow fed through the supply flue incorporating the bent portion, the tubular insets undergo particularly heavy wear because the gas flow velocity therein is equal to that inside the heat-exchange tubes. In this case the tubular insets must be replaced quite frequently consuming much labour because the number of heat-exchange tubes in such an air heater as that used in the boiler of 500-MW power unit amounts to about 200,000 pieces.
In addition, the manufacturing procedure of the air heater as a whole is rather labour-consuming since the tubular insets are sunk in the tube plate to permit butt jointing thereof with the heat-exchange tubes, and, hence, the assembly and welding of the air heater are considerably complicated.
There is likewise known in the art a device for heating air by flue gases in steam boilers and furnaces, comprising a supply flue, adjacent sections each consisting of heat-exchange tubes attached to tube plates, and a device for protection of the tubes against abrasive wear disposed inside the supply flue, which is the closest analogue to this invention (cf. the magazine TEPLOENERGETIKA (HEAT ENGINEERING) (in Russian), ENERGOIZDAT Publishers, Moscow, No. 12, 1981, pp 18 through 21).
The device for protection of tubes against abrasive wear used in the prior-art air heater comprises sheets located in the bend of the supply flue and intended to direct the flue gas flow to the heat-exchange tubes equipped with tubular insets attached to a tube plate.
The flue gas flow through the supply flue incorporating a bent turn portion is turned by the sheets through an angle of about 90 deg and is sent to the heat-exchange tubes along the axes thereof, with the result that the rate of abrasive wear of the heat-exchange tubes is reduced.
The tubular insets mounted on each heat-exchange tube protect the tubes against abrasive wear by localizing said wear directly in the insets. Since the flue gas flow velocity in the tubular insets is as high as that in the heat-exchange tubes because of equal cross-sectional areas, the insets proper are exposed to heavy abrasive wear.
It should be noted that centrifugal forces produced by the turn of the flue gas flow in the supply flue drive hard particles toward the guide sheets where they accumulate at guide sheet parts which are terminal with respect to the direction of gas flow. Concentration of hard particles in said areas leads to heavier wear of the tubular insets located directly beside the terminal parts of the guide sheets, and, hence, to a higher rate of wear of the device as a whole. To preclude accumulation of hard particles in the foregoing areas, they must be evenly distributed over the entire cross section of the supply flue, which is practicable only by installing a great number of guide sheets therein. Yet, such design of the device increases the metal requirements of construction, complicates the assembly procedure and renders the device irrepairable.
In addition, the tubular insets exposed to heavy wear require frequent replacement, and the costs of manufacture, removal and installation thereof are high.
Furthermore, to provide for dependable protection of the heat-exchange tubes against abrasive wear, the tubular insets installed on the tube plate must be accurately aligned axially with the heat-exchange tubes so that the gas flow should smoothly enter each heat-exchange tube.
Should the tubular insets be displaced from the heat-exchange tubes even by 0.5 to 1.0 mm, the efficiency of protection of the heat-exchange tubes against abrasive wear decreases abruptly.